Restriction endonuclease mapping of mouse mitochondrial DNA is currently in progress. The three sites for Hpa I and the five sites for Hin II have been located with respect to the origin of replication. Mapping of the sites for Bam I and Sal I is currently in progress. When their cleavage sites have been located, the smallest piece containing cohesive termini which includes the origin of replication will be cloned in E. coli as a recombinant with the plasmid pBR313. This cloned mitochondrial DNA fragment will be used to carry out the base sequence analysis of the replication origin. The map is being constructed with cultured L cell DNA. This map, in conjunction with the known maps for Hin III and Eco RI, will be adequately detailed to allow comparison with mitochondrial DNA from mouse sources. Restriction mapping of human mitochondrial DNA is also being carried out. The novel mitochondrial DNA structures in certain neoplastic cells lends interest to comparisons of homology between DNAs from normal and neoplastic cells. Work is continuing on the location of the covalent ribonucleotides in mouse mitochondrial DNA, on the application of agarose gel electrophoresis to analysis of DNA structure and replication, and on the mechanism of action of type II restriction endonucleases.